The field of the invention is magnetic resonance imaging and in particular a local coil for use in magnetic resonance imaging.
Magnetic resonance imaging (“MRI”) provides images, for example, of a human patient, by detecting faint signals from precessing hydrogen protons under the influence of a strong magnetic field (termed the B0 field) after a radio frequency excitation.
The quality of the image produced by MRI is strongly dependent upon the strength of the received signal. For this reason, it is known to use radio frequency receiving coils placed in close proximity to the volume being imaged. Such coils are called local coils.
A particular type of local coil, termed a “phased array coil”, provides a series of small loops that together define a surface covering a region of interest. Magnetic flux from NMR signals within the region of interest passes through the surface defined by the loops and is detected separately at each loop. The relatively smaller area of the loops provide improved signal-to-noise ratio, better spatial localization for certain MRI techniques, and extended coverage of the patient made possible by switching different sets of loops into communication with limited MRI machine inputs.
In order to ensure continuous coverage of a region of interest, the constituent loops of a phased array coil are normally placed in close proximity. This proximity promotes a coupling between the loops that reduces signal-to-noise ratio in the received NMR signal. For this reason, the coupling is normally reduced by one of several isolation techniques.
In a first decoupling method, a slight but precise overlap of the loops is created to promote a “flux sharing” of countervailing flux between the loops sufficient to decouple the coils from each other. This technique is described in U.S. Pat. No. 5,256,971 assigned to the assignee of the present invention and hereby incorporated by reference.
Decoupling coils by flux sharing can be difficult in practice because the amount of overlap must be precisely controlled and can change depending on the imaging environment. Accordingly, second methods of decoupling may be used instead or in addition to flux sharing, such methods including the use of networks of decoupling capacitors and/or the use of preamplifiers with input impedances selected to reduce current flow in the loops.
Despite many advantages of phased array coils, uniform sensitivity over the surface covered by the loops is hard to obtain because of a drop-off in sensitivity of the loops at their edges. In coils employing symmetric patterns of loops, the drop off may be most pronounced at the center of body structure where sharp imaging and high signal-to-noise ratio signals are required.